The invention relates to a drive device with an electric drive motor, with a housing, with at least one shaft driven by the drive motor and with compensating means to compensate for the axial play of the shaft.
In the following a shaft driven by a drive motor is understood to mean an output shaft driven by the drive motor through a gear, specifically through a worm gear, as well as an armature driven directly by the drive motor.
Drive devices of this type find an application particularly in the automotive industry as servo motors for seat adjustments, steering column adjustments, window regulators, sliding sunroofs or similar, and as drive devices for windshield wiper systems. The drive devices normally have a worm gear attached to the drive motor, which advantageously has small dimensions, can transmit high levels of power and is self-braking.
Specifically because of the tapered flanks of the worm shafts and the worm gear in a worm drive of this type, an axial force acts on the armature of the drive motor during operation of the drive device, as well on the output shaft of the worm drive. The direction of the axial force depends on the direction of rotation of the armature. With a change in direction of the drive motor or a change in external load the direction of the axial force on the armature as well as on the output shaft is reversed because the worm gear is loaded in the opposite direction. Because of manufacturing tolerances in the individual components of the drive mechanism and because of operating wear in the area of the axial mounting of the shafts, relatively large, undesirable play can develop in the various shafts. As a consequence of axial play of this type, abrupt starting motions and irritating noises result when the drive motor reverses direction.
In addition, it is difficult to achieve precise positioning when starting or reversing a window that is being opened, for example, or a sliding sunroof.
Furthermore, noticeable relative motions can result if the servo motor is employed as a seat back adjuster, for example, and the driver perceives the axial play as relative motion even when the motor is not running.
This phenomenon is additionally known in the case of steering column adjustments.
From DE-OS 23 12 395 a drive device of the type described above is known which has a small wedge-shaped or vee-shaped block at the open end of the shaft, operating axially on the shaft as a compensating means to compensate for the play in a shaft. The small block is impelled by means of a coil spring. The disadvantage of a device of this type is that corresponding protrusions, which are expensive to manufacture and take up additional installation space, are required on the housing to locate the small block and the accompanying coil spring.
The object of the present invention is therefore to design and develop a drive device of the type named at the beginning so that it is constructed as simply as possible and compensates reliably for the axial play in the shaft.
This object is accomplished by locating a tapered thrust bearing which can be expanded radially against a spring force on the shaft as a compensating means, with two contact surfaces tapering radially inward toward the shaft, by the shaft having an annular projection which corresponds to one of the contact surfaces and by the housing having an annular collar which corresponds to the other contact surface, where the expanded tapered thrust bearing is positioned between the projection and the annular collar under pre-load.
What is accomplished under the invention is that because of the expanded tapered thrust bearing, which is under pre-load and which bears against the housing, the shaft is pressed against the annular collar with an axial force by means of the tapered contact surfaces. The axial force compensates for the axial play. Since the expanded tapered thrust bearing is positioned between the housing and the annular projection on the shaft, almost no additional installation space is required in order to locate the compensating means. Advantageously under the invention, the number of individual components in the compensating means is kept down to only the expanded tapered thrust bearing, since the projection can be executed in one piece with the shaft and the annular collar in one piece with the housing. A substantial advantage of the invention is that because of the pre-loaded expanded tapered thrust bearing, axial play is compensated for when the drive motor is running as well as when it is stationary.
During assembly of the drive device, the tapered thrust bearing positioned between the annular collar and the projection on the shaft is expanded, whereby the axial portion of the pre-load force from the expanded tapered thrust bearing acting on the shaft through the two tapering contact surfaces impinges on the shaft with an axial force that compensates for the axial play.
Advantageously, the contact surfaces run symmetrically at an angle of about 15xc2x0 to the plane formed by the tapered thrust bearing, where the surfaces on the annular collar and the projection corresponding to the contact surfaces have a matching taper. This achieves an even application of the pre-load force from the tapered thrust bearing against the shaft on the one hand and against the housing on the other hand. This counteracts any tendency on the part of the tapered thrust bearing to become wedged between the annular collar and the projection on the shaft.
In order to ensure that the tapered thrust bearing is sufficiently expanded, it is advantageously configured to be slotted. In addition, provision can be made for the tapered thrust bearing to have slot-like recesses in the area of its inner circumference which permit additional elastic expansion and contraction of the tapered thrust bearing.
In an advantageous aspect of the invention, the tapered thrust bearing has a slotted spring clamping wire. By this means, a higher spring constant in particular is achieved, and the tapered thrust bearing can be placed under a higher pre-load.
Advantageously, the spring clamping wire is located in a groove running around the circumference of the tapered thrust bearing. This allows the tapered thrust bearing to be preassembled jointly with the spring clamping wire, which results in substantial simplification of final assembly of the drive device.
In order to prevent slipping or twisting of the spring clamping wire, advantageous provision can be made for the groove to have a transverse rib in the area facing away from the slot on the tapered thrust bearing to locate the slot in the spring clamping wire.
In another further development of the invention, the shaft has an annular groove-like recess in which the tapered thrust bearing is retained by positive engagement. This has the advantage that the shaft can be preassembled together with the tapered thrust bearing and loosening is prevented because the tapered thrust bearing is positively retained on the shaft.
In the case of an especially preferred aspect of the invention, the tapered thrust bearing is made of plastic, specifically from a polyamide, where the plastic has an anti-friction coating specifically of graphite, molybdenum disulfide or similar materials, or, respectively, contains graphite, molybdenum disulfide or similar materials. The effect of this is that the two contact surfaces on the tapered thrust bearing have good friction characteristics with respect to the projection on the shaft on the one hand and the annular collar on the gear housing on the other.
Advantageous provision can be made under the invention for the projection to be located on a gear wheel, specifically a worm wheel on the shaft. This dispenses with an annular projection which has be specially located on the shaft.
In an additional development of the invention, the projection is made from a plastic, specifically from polymethylene oxide. A plastic of this kind exhibits advantageous friction properties specifically with respect to a tapered thrust bearing of polyamide. It is also conceivable that the annular groove-like recess is configured as a plastic part in one piece with the projection, which eliminates machining of the shaft.
In another advantageous aspect of the invention, the annular collar is located on a cover of the housing, specifically a zinc die-cast cover. With the location of the cover on the housing, the annular collar is pressed against the contact surface of the tapered thrust bearing facing the annular collar when the drive device is assembled, whereby the tapered thrust bearing is expanded and is positioned between the projection and the annular collar under pre-load.